The proposed research will be an investigation of the mechanism of Na ion transport across rabbit urinary bladder. Conventional and ion specific microelectrodes will be used to measure apical and basolateral membrane potentials and conductances as well as intracellular ion activities at various Na ion transport rates of the urinary bladder. The polyene antibiotic nystatin will be used to increase apical conductance to that of free solution for monovalent ions. Measurements of apical and basolateral membrane potential and conductance while varying external ion composition will allow calculation of the selective permeability of both membranes. The intracellular ion specific microelectrodes will facilitate the calculation of the passive permeabilities as well as yield insight into the mechanism of Na ion entry across the apical membrane and extrusion across the basolateral membrane, i.e., passive diffusion or active transport. These experiments will also allow a study of the postulated negative feedback system between intracellular Na ion and apical membrane conductance. Nystatin will be used to eliminate the apical membrane as a barrier between mucosal solution and cell cytoplasm. By using radio-isotopes, this method will be used to study the passive permeability and coupling ratio of the Na ion - K ion pump located in the basolateral membrane. The results will contribute to the further development of a model of Na ion transport across tight epithelia.